Reversible changes of absorption, induced by flashes of strong light were studied in various photosynthetic organisms. The technique used allowed to include the red and infrared spectral regions into this study. In the short wavelenght region the results showed good agreement with earlier observations largely made with continuous actinic illumination. Though significant differences between various types of plants appear to exist, a decrease in absorption in the red region, characterized by several maxima, was generally observed, the most pronounced effect occurring at 705 my. An increase of absorption was observed between 720 and 820 my.